Graphite and carbon materials, especially reinforced carbon-carbon composite substrate materials, are subject to many applications in modern industry, particularly in the aerospace and aviation fields. However, such materials, when unprotected, are subject to degradation at elevated temperatures. Since many applications involve high temperatures, resistance to high temperature and thermal shock are often required properties of the material.
Reinforced carbon-carbon composite substrates are generally constructed of fibers and bound by a carbon matrix, resulting in a material having excellent structural properties. Generally, carbonaceous fibers such as polyacrylonitrile, rayon or pitch-based fibers are utilized. Carbon-carbon impregnation materials generally are phenolic, furfuryl or pitch-based materials. However, the use of a specific substrate material is not a limitation upon the present invention.
Graphite and carbon materials, including reinforced carbon-carbon composites, are subject to degradation, such as oxidation, when utilized in high temperature environments in the presence of oxygen. Generally, an unprotected graphite or carbon material will begin to oxidize at temperatures in excess of about 650.degree. F. in air. Therefore, in order to effectively utilize these materials in high temperature applications, it is necessary to provide protection from degradation, including oxidation. Accordingly, a need exists for a composition of matter and method for forming protective coatings on graphite and carbon materials, and especially for reinforced carbon-carbon composite materials.
One indication of high temperature degradation resistance is the percent weight change of the coated substrate or part that is exhibited over a period of exposure in an elevated temperature environment containing oxygen. It is desirable for a coated substrate not to exhibit a significant weight change after exposure to high temperatures in oxygen containing environments since this could affect the performance of the device incorporating such materials.
Accordingly, a need exists for a method and composition of matter for forming protective coatings on carbon substrates that provide resistance to degradation at elevated temperatures of up to about 3000.degree. F. and to improve said substrates resistance to thermal shock. Further, a need exists for a method and composition of matter for forming protective coatings on reinforced carbon-carbon composite substrates wherein protection from degradation is provided at elevated temperatures where temperature cycling frequently occurs, thereby subjecting the substrate to varying temperatures between about 3000.degree. F. and below 0.degree. F. Finally, improved coatings which are resistant to cracking and crazing, which adversely affect their ability to protect the substrate, are needed.